Captions and subtitles are added to video to describe or enhance video content using the written word. Subtitles are typically a transcription or translation of the dialogue spoken by the actors in a video. Captioning is intended to convey information for listening-impaired viewers, and typically contains non-speech information such as sound effects in addition to dialogue text. Captions and subtitles may be “open” or “closed.” Open captions/subtitles are part of the video signal and cannot be turned on or off by the viewer, whereas closed captions/subtitles can be turned on or off based on user selection.
A number of technologies have been developed for delivery of captions and subtitles. Closed Caption (CC) technology has been widely used in the United States, and PAL Teletext has been used primarily in Europe. These caption technologies involve encoding text data in a non-visible portion of the video signal. In DVDs, subtitle data is stored as bitmaps, separate from the main video data. To generate an output video signal that includes subtitles, the main DVD video data is decoded, and then subtitle data is added to the main video data as a subpicture data stream.
Although subtitles and captions are intended to improve the viewer's understanding of the accompanied video, subtitles and captions can often be difficult to read. Difficulty in reading on-screen text can be attributed to a number of causes: the display in which the subtitles are displayed may be too small; the viewer may have poor eyesight or may be too great a distance from the display; the viewer may have difficulty with the language in which the text is displayed; the text may be poorly authored; the text may change too quickly; or the background on which the text is rendered may be of a color that makes reading the text difficult. Therefore, advancements in the readability and accessibility of on-screen text are required.